As is known, electromagnetic switching devices are often used to electrically couple a power source to a load such as an electrical motor or the like. The electromagnetic switching device includes both fixed and movable electrical contacts, as well as, an electromagnetic coil. Upon energization of the electromagnetic coil, the movable contact engages the fixed contact so as to electrically couple the power source to the load. When the electromagnetic coil is de-energized, the movable contact disengages from the fixed contact thereby disconnecting the load from the power source. However, as the contacts are separated, current continues to flow therebetween resulting in an arc between the contacts if minimum arc voltages and arc currents are present. Repeated or continued arcing between the contacts interferes with the ability of the contacts to conduct electricity and may cause the surface of the contacts to become eroded, pitted, or develop carbon build-up. Further, in circuits with high voltage sources, elimination of the continued arcing between the contacts may require special contact configurations, arc chutes, vacuum sealed devices or gas back filled devices. These arc-eliminating devices increase the size and weight of the switching devices. Hence, it is highly desirable to minimize or eliminate the potential for arcing between the contacts of a switching device without resorting to use of these arc-eliminating devices.
Various devices have been developed to minimize the arcing that may occur between the contacts of a switching apparatus such as an electromechanical switching device. By way of example, Kawate et al., U.S. Pat. No. 5,536,980 discloses a high voltage, high current switching apparatus that incorporates various protector devices that are used in the event of a circuit malfunction. The switching apparatus incorporates a single pole, double throw switching device and a solid state power switch. When the coil of the switching device is energized, the contact arm of the switching device moves into engagement with a first load contact that is operatively connected to a load. When the coil is de-energized, the contact arm of the switching device moves into contact with a second contact which is operatively connected to the gate of an IGBT switch. The collector of the IGBT switch is interconnected to the first load contact. Upon energization of the coil switching device, the movable contact moves toward the first load contact and the switch is turned on. Since the time required for the movable contact to move from the second contact to the first load contact is much greater than the switch turn-on time, the switch will be on prior to engagement of the movable contact with the first load contact. As a result, arcing between the movable contact and the first load contact is eliminated.
When the coil is de-energized, the movable contact starts to move away from the first load contact toward the second contact. Since the IGBT switch is already on, all of the current will flow through the IGBT switch until the movable contact engages the second contact. When the movable contact engages the second contact, the IGBT switch is turned off thereby turning off the load.
While the switching apparatus disclosed in the Kawate et al., '980 patent minimizes the arcing between the contacts of a switching device during switching, the circuit disclosed therein has certain inherent problems. More specifically, the circuit disclosed in the '980 patent functions to switch the load between the power source and ground. As such, the load may remain hot after the switching process thereby resulting in a potential of shock hazard from the load for a user. Further, the switch remains on whenever the first load contact of the switching device is closed. As a result, the circuit disclosed in the Kawate et al., '980 patent dissipates a significant amount of heat and utilizes a significant amount of power.
Therefore, it is a primary object and feature of the present invention to provide a bypass circuit that minimizes the arcing between the contacts of a switching device during the opening thereof.
It is a further object and feature of the present invention to provide a bypass circuit for minimizing the arcing between the contacts of a switching device that dissipates less heat and utilizes less power than prior bypass circuits.
It is a still further object and feature of the present invention to provide a bypass circuit for minimizing the arcing between contacts of a switching device that is simple and inexpensive to implement.
It is a still further object and feature of the present invention to provide a bypass circuit to eliminate arcing between contacts of a switching device that may be utilized with any switching device regardless of contact configuration and without the use of additional contacts for controlling the bypass circuit.
In accordance with the present invention, a device is provided for preventing arcing between the contacts of an electromechanical switching device as the contacts of the switching device are opened. The switching device includes a coil for controlling the opening and closing of the contacts. The device includes a coil suppression circuit connected in parallel with the coil. The coil suppression circuit dissipates the energy stored in the coil in response to the de-energizing of the coil. The device further includes a solid state switch having a gate operatively connected to the coil suppression circuit. The solid state switch is also connected in parallel with the contacts. The switch is movable between an open position for preventing the flow of current therethrough and a closed position in response to the dissipation of energy by the coil suppression circuit.
The coil suppression circuit includes a first zener operatively connected to the coil. The first zener diode provides a reference voltage in response to the de-energizing of the coil. A driver has an input operatively connected to the first zener diode and an output operatively connected to the gate of the solid state switch. The driver closes the solid state switch in response to a reference voltage across the first zener diode. The driver may also include a timing device for closing the solid state switch for a predetermined period of time.
The coil suppression circuit may also include a second diode operatively connected to the coil in series with the first zener diode. The first zener diode is biased in a first direction and the second diode is biased in a second opposite direction.
Alternatively, the driver may include a transformer. The transformer has a primary side operatively connected to the coil suppression circuit and a secondary side interconnected to the gate of the solid state switch. The transformer transfers electrical energy from the coil suppression circuit to the gate of the solid state switch. A zener diode may be connected in parallel to the second side of the transformer and the transformer has a preferred turn ratio of 1:1.
The first solid state switch includes a collector operatively connected to a first contact and an emitter. In addition, the device may include a second solid state switch. The second solid state switch may include a collector operatively connected to the emitter of the first solid state switch, an emitter operatively connected to a second contact of the switching device, and a gate operatively connected to the gate of the first solid state switch. A first diode extends between the collector and the emitter of the first solid state switch. The first diode is biased in a first direction. A second diode extends between the collector and the emitter of the second solid state switch. The second diode is biased in a second direction.
In accordance with a further aspect of the present invention, a bypass circuit is provided for preventing arcing of electrical energy passing between first and second contacts of an electromagnetic switching device having a coil wherein the contacts open and close in response to energization of the coil. The bypass circuit includes a first switch connected in parallel with the contacts of the electromagnetic switching device. The first switch is movable between a closed position with the contacts open and an open position with the contacts closed. An actuation circuit interconnects the coil and the first switch. The actuation circuit closes the first switch in response to de-energization of the coil.
The actuation circuit includes an energy dissipation device operatively connected to the coil to dissipate a portion of the energy released by the coil as the coil is de-energized. A driver interconnects the energy dissipation device and the first switch. The driver closes the first switch in response to a portion of the energy dissipated by the energy dissipation device. The energy dissipation device may take the form of a zener diode. The driver may take the form of a transformer. The transformer has a primary side operatively connected to the energy dissipation device and a secondary side operatively connected to the first switch.
It is contemplated that the electrical energy passing between the contacts have an AC waveform. As such, the bypass circuit may also include a second switch operatively connected to the actuation circuit and connected in parallel with the contacts of the electromagnetic switching device. The second switch is movable between a closed position with the contacts open and an open position with the contacts closed.
In accordance with a still further aspect of the present invention, a bypass circuit is provided for preventing arcing of electrical energy passing between first and second contacts of an electromagnetic switching device having a coil wherein the contacts open and close in response to energization of the coil. The bypass circuit includes a first switch connected in parallel with the contacts of the electromagnetic switching device. The first switch is movable between an open position and a closed position. An energy dissipation device is operatively connected to the coil to dissipate a portion of the energy released by the coil as the coil is de-energized. A driver interconnects the energy dissipation device and the first switch. The driver closes the first switch prior to the opening of the contacts in response to the portion of the energy dissipated by the energy dissipation device.
The driver may take the form of a transformer having a primary side operatively connected to the energy dissipation device and a secondary side operatively connected to the first switch. If the electrical energy passing between the contacts has an AC waveform, the bypass circuit may include a second switch operatively connected to the driver and connected in parallel with the contacts of the electromagnetic switching device. The second switch is movable between an open position and a closed position. The driver closes the second switch prior to the opening of the contacts in response to the portion of energy dissipated by the energy dissipation device.